Nationality initialization fixes

svornostrequested to merge
svornost/fc-pregmod:pregmod-master into pregmod-master
  1. A handful of type fixes for initNationalities
  2. Preserve slave trade data in NGP (fixes #2858 (closed) per Pregmodder's comment)

Note that existing games have already lost the customization flag, and will still lose their custom nationalities in NGP.

Edited by svornost

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